Thermoplastic or thermosetting plastic materials, including reinforcing fibers, such as glass filaments, have been proposed heretofore as suitable construction materials for solar energy absorbing panels and for hot liquid storage tanks. While such plastic materials are considerably cheaper than copper or stainless steel for such panels or tanks, their tensile strength is generally low at near boiling-water temperatures which they must sustain in service. Further, most of such materials become more soluble in the heat absorbing liquid, generally water or a typically an anti-freeze liquid, such as alcohol or ethylene glycol, at elevated operating temperatures. While thermoset materials, such as epoxys or polyesters, and particularly those bonded with glass fibers, are quite satisfactory, such material is expensive when cast or formed in a panel as large as 4 ft. by 8 ft. that is thick enough to be supported at a few points to tilt or elevate it. Such limited support is necessary to avoid serious overloads on a building roof if a conventional steel frame or a wooden support pad is required to support the entire solar panel. The problem is even more severe in a passive solar system. In passive systems the liquid storage tank is either positioned above the solar panel, so that both tank and panel must be separately supported, or as taught in my U.S. Pat. Nos. 4,137,964 and 4,050,508, the storage tank and panel are mounted back-to-back in a single support box. In either case, the total load of both solar panel and storage tank further stress any mounting box and support means.
In addition to the problems noted above with regard to the solar panel or storage tank, a metal support box adds to both the weight and the overall cost of the installation. Where the panel and tank are made of thermoplastic or rubber, the containers themselves have little or no rigidity to resist buckling or compression, and generally thermoplastic materials have reduced tensile strength at elevated temperatures. Accordingly, a light weight, inexpensive container or support box for a solar panel or a storage tank, or both (as in the case of a back-to-back assembly) is highly desirable both to reduce the overall initial cost of solar water heating and to limit cost for maintenance over an extended life of from ten to twenty years, as projected for an adequate passive solar water heating system.